Steel-reinforced concrete is a remarkably versatile and ubiquitous material. It is easily formed from a cement-based slurry which hydrates with water, without extensive volume change, to form solid, compressively strong structures of virtually unlimited size and shape. The structural weakness of concrete in tension is compensated by relatively inexpensive steel reinforcement, which fortuitously approximates the thermal expansion of concrete over a broad temperature range. Unfortunately, steel reinforcement is subject to corrosion (rusting), even when embedded in the high-pH, passivating environment of the hydrated cement paste. The pressure from the increased volume of the confined iron oxide corrosion product spalls and cracks the concrete, destroying its functional integrity long before the end of its potential design life. Estimates of the repair cost for existing highway bridges in the U.S. have been put at over $50 billion, and $1 to $3 trillion for all concrete structures M. Fickleburn, “Editorial”, Materials and Structures Journal, RILEM, Vol. 23, No. 137, p. 317(1990).
Except for its tendency to corrode, and its poor mechanical interface with the cement matrix, steel-reinforced concrete has many desirable attributes which have made it a dominant “Infrastructure” material for roads, bridges, buildings and dams throughout the world. An enormous design, manufacturing and construction industry has developed which is efficient and skilled in its use and application. In the absence of corrosion, its behavior is relatively predictable and reliable. Conventional steel reinforcement is relatively inexpensive, particularly in comparison to inherently expensive alternatives such as Fiber Reinforced Plastic (FRP) and stainless steel.
Corrosion of steel reinforcement is greatly accelerated in the presence of chloride ions, which catalyze iron oxidation. Even if not present in the original concrete, chlorides can penetrate throughout the pore structure of the concrete when the external surfaces of the concrete are exposed to deicing salts or marine environments [J. A. Gonzalez, et al., “Some Considerations On The Effect Of Chloride Ions On The Corrosion Of Steel Reinforcements Embedded In Concrete Structures”, Magazine Of Concrete Research, Vol. 50, pp. 189-199 (1998)].
A wide variety of approaches has been attempted to overcome the problem of corrosion in reinforced concrete. For example, steel reinforcement with a relatively expensive epoxy coating is commonly used in bridges and highways. However, the soft epoxy coating is easily damaged during transportation and placement. The epoxy coating also prevents bonding of the rebar with the cement matrix, and itself tends to debond from the steel surface because its bond can be thermodynamically unstable in the alkaline cement environment. When debonded from the steel surface, the epoxy layer may be associated with underfilm and crevice corrosion. [Weyers, et al., “Estimating the Service Life of Epoxy-Coated Reinforcing Steel”, ACI Materials Journal, pp. 546-557 (1998)].
Another expensive approach has been the use of reinforcing materials which do not corrode. Stainless steel reinforcing bars meeting appropriate specifications (ASTM A955 and BS 6744) have been used in selected applications [F. N. Smith, et al., “Stainless Steel Reinforcing Bars”, Proc. Conf, Conference on Understanding Corrosion Mechanisms in Concrete, MIT, Cambridge, Mass., 27-31 Jul. 1997; F. N. Smith: “The Use Of Stainless Steel For Concrete Reinforcing Bars Is Gaining Momentum”, Stainless Steel World, Vol. 10, No. 6, August 1998, p. 52], but unfortunately are relatively expensive and difficult to work with, and consume strategic materials. Fiber-reinforced plastic (FRP) reinforcing materials, which are even more expensive, are being developed as inert, corrosion-resistant reinforcement for concrete in marine and high-deicing-salt environments. [R. Masmoudi et. al., “Flexural Behavior of Concrete Beams Reinforced with Deformed Fiber Reinforced Plastic Reinforcing Rods”, V. 95, pp. 665-676 (1998); Neale, K. W., and Labossiere, P., eds. Advanced Composite Materials in Bridges and Structures, Proceedings, First International Conference on Advanced Composite Materials in Bridges and Structures, Sherbrooke, Canada, published by the Canadian Society for Civil Engineering, 1992, 700 pp.; A. Nanni, et al., eds., Fiber Reinforced Plastic Reinforcement for Concrete Structures, SP-138, American Concrete Institute, Farmington Hills, Mich., 1993, 977 pp.; Saadatmanesh, H. and Ehsani, M. R., eds., Composites in Infrastructure, Proceedings, First International Conference on Composites in Infrastructures, Tucson, Ariz., 1996, 600 pp.]. FRP reinforcements typically comprise carbon, aramid and/or glass fibers embedded in a resin matrix, to provide a high tensile strength-to-weight ratio [O. Chaallal, et al., “Physical and Mechanical Performance of an Innovative Glass-Fiber Reinforced Plastic Rod for Concrete and Grouted Anchorages, ”Canadian Journal of Civil Engineering, V. 20, No. 2,1993, pp. 254-268.]. However, in spite of their potentially high strength based on the strength of the individual fibers, radial force transmission among adjacent fibers may be poor, and failure in fiber reinforced plastic composites may occur at relatively low strains (about 1-3 percent). Their tendency for sudden, brittle failure, particularly at high strain rates, may present safety problems under current reinforced concrete design criteria [Banthia, et al., “Impact Resistance of Concrete Plates Reinforced with a Fiber Reinforced Plastic Grid”, ACI Materials Journal, pp. 11-18 (1998)]. Unlike steel, FRP reinforcement also has a significantly higher radial thermal expansion than concrete, posing compatibility problems, and inherently poor adhesion to the cement matrix.
Because of the poor corrosion characteristics of steel reinforced concrete, there is significant opportunity to improve the overall properties of steel reinforced concrete, if improved interfaces can be provided [X. Fu, et al., “Effects Of Water-Cement Ratio, Curing Age, Silica Fume, Polymer Admixtures, Steel Surface Treatments, And Corrosion On Bond Between Concrete And Steel Reinforcing Bars”, ACI Materials Journal, pp. 725-734 (1998)]. It is an object of the present invention to improve the mechanical properties and/or the durability, of steel reinforced concrete could be improved if the low density, highly porous interface zones adjacent the steel reinforcement surfaces could be replaced by dense, strong, adherent layers.
Accordingly, there is a need for improved concrete reinforcement, as well as functional cement-steel interfaces, which prevent corrosion of the steel reinforcement and resultant premature deterioration of steel reinforced concrete structures. There is also a need to improve anchoring and transmission of mechanical forces between the steel reinforcement and the cement matrix, in order to improve the performance and durability of the concrete. It would also be desirable to prevent percolation of corrosion-causing chloride salts along the surfaces of steel reinforcement in cured concrete structures.
There is also a need for functional interfaces which maximize underlying microstructure and hydration transformations in concrete, to optimize durability and macro-mechanical properties.